1. Field of the Invention
The present invention relates to the field of Chemical Mechanical Polishing (CMP), and more particularly, to methods and systems for separating large particles and foreign matter from an abrasive polishing slurry prior to polishing workpieces.
2. Description of the Related Art
Chemical Mechanical Polishing is a method of polishing materials, such as semiconductors substrates, to a high degree of planarity and uniformity. The process is used to planarize semiconductor slices prior to the fabrication of semiconductor circuitry thereon, and is also used to remove high elevation features created during the fabrication of microelectronic circuitry on the substrate. One typical chemical mechanical polishing process involves rotating a semiconductor wafer on a polishing pad, applying pressure through a rotating chuck, and supplying an aqueous polishing slurry containing an abrasive polishing agent to the polishing pad for abrasive action. Specifically, the abrasive agent is interposed between the wafer and polishing pad to planarize the surface.
Generally, abrasive polishing agents used in chemical mechanical slurries include particles of fumed silica, colloidal silica, cerium oxide and/or alumina particles. Fine silica particles are often used as the polishing agent in a CMP process, because silica particles exhibit good dispersion and uniformity in average particle dimension. The fine silica particles are dispersed in a dispersion medium, such as water, and used as a silica suspension.
The slurry and material removed from the semiconductor wafer during a polishing process form a waste stream that is commonly disposed of as industrial waste because reuse of the polishing slurry that contains large-sized polishing refuse or aggregation may cause damage to the polished surface. However, the disposal of dissolved or suspended solids in the industrial waste stream has become a relevant environmental issue due to strict local, state and federal regulations. As such, it would be desirable to provide a process and apparatus to remove abrasive components from the waste stream for possible reprocessing and reuse as a chemical mechanical slurry.
Conventional techniques for reclamation of water and separation of large particles typically greater than 3-4 microns in diameter include reverse osmosis filtration, microfiltration, centrifugation using a screen bowl centrifuge or electrophoresis. However, such techniques are commonly limited to batch processing or have low throughput volumes. Further, these techniques are not readily adapted to high volume, continuous service. Also, these conventional methods do not attain sufficient removal of larger diameter particles that otherwise can cause surface damage to the semiconductor wafers including scratches, pits and other flaws.
U.S. Pat. No. 4,634,536 describes a method and process using a screen bowl centrifuge for separation. However, separation is limited to batch processes and further limited by clogging of the screen in the centrifuge as solids tend to build up on the screen.
Accordingly, there is a need for an improved separation process and system for polishing slurries wherein the process and system provide a high volume continuous flowthrough and ensure continuity in particle size thereby reducing the risk of damage to the polished surface incident to the presence of larger diameter particles and agglomerated solids.
The present invention relates to a process and system for treatment of CMP slurry compositions to remove overlarge solids therefrom, so that the CMP operation is correspondingly enhanced in operational efficacy.
In one aspect, the present invention relates to a process and system to remove particles having a diameter greater than about 0.5 microns from an abrasive slurry thereby ensuring reduced scratching of a surface substrate during a subsequent polishing process.
Another aspect relates to a closed loop slurry supply system for recovery and reuse of components of an aqueous chemical mechanical polishing abrasive slurry thereby reducing the cost of the chemical mechanical polishing process.
Yet another aspect of the present invention relates to a recovery process that reduces the adverse environmental impact of the polishing process.
Still another aspect of the present invention relates to a continuous method and system of separation operable at suitable flow rates to support high volume flow of a polishing slurry to a polishing apparatus of the type generally used in the semiconductor industry, and/or waste produced by such a polishing apparatus.
The present invention in one aspect relates to a method for continuous separation and removal of potentially damaging particles from a polishing slurry prior to a chemical mechanical polishing process utilizing such slurry, the method comprising:
filtering a polishing slurry comprising at least one abrasive polishing agent through a filter having a pore size not greater than 25 microns;
introducing the filtered polishing slurry into a solid bowl, sedimentation-type centrifuge comprising a vertical stack of thin discs;
separating abrasive polishing particulates having a particle size greater than about 0.5 micron from the filtered polishing slurry and continuously ejecting the particulates through nozzles on the solid bowl sedimentation-type centrifuge to yield a product slurry; and
continuously removing the product slurry from the centrifuge having abrasive particles of about 0.5 microns and less, to provide a polishing slurry for chemical mechanical polishing.
According to another embodiment of the present invention, a polishing agent separation system comprises a filter means for removing particles larger than 25 microns, and a means for separating particles larger than 0.5 microns from the polishing slurry.
Preferably, the solid bowl, sedimentation-type centrifuge is equipped with a disc-type bowl having a double conical solid holding space which is fitted with nozzles at the periphery of the bowl. Separation of larger abrasive particles from the aqueous polishing slurry takes place in the disc stack, wherein the solids slide down into the double-conical solid holding space and are continuously discharged through the nozzles.
The separation methods of the present invention may be used for processing new polishing slurries and recovered polishing slurries used in a previous polishing process to ensure a non-damaging polishing slurry that is essentially devoid of foreign matter or aggregates that exceed 0.5 microns.
The aqueous polishing slurries treated according to the present invention act to mechanically and chemically abrade and remove the surface of the workpiece to a desired extent.
Another embodiment of the present invention is directed to a method for separating and removing potentially damaging particles from a waste polishing slurry recovered from a chemical mechanical polishing process, the method comprising:
filtering the waste slurry comprising abrasive polishing particulates and waste debris through a filter having a pore size not greater than 25 microns;
introducing the filtered waste slurry into a solid bowl, sedimentation-type centrifuge having a vertical stack of thin discs;
separating abrasive polishing particulates and waste debris having a particle size greater than about 0.5 micron and ejecting same through nozzles on the periphery of the solid bowl sedimentation-type centrifuge yielding a purified polishing slurry; and
continuously removing the purified polishing slurry from the solid bowl sedimentation-type centrifuge, wherein the polishing slurry comprises particles having a diameter not exceeding about 0.5 microns to provide a polishing slurry that reduces damage to polished surface during a subsequent chemical mechanical polishing process, relative to corresponding use of the waste slurry.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the present invention.